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Posts Tagged ‘Henson Creek’

In part two of my Henson Creek fossil series on this blog I will be discussing Ostrea compressirosta. This bivalve is a member of the subclass Pteriomorpha like the Cucullaea Gigantea mentioned in my previous post, though this one adhered to the surface of the seafloor as opposed to burrowing into the sediment. Both were filter feeders that strained their nourishment from the water using a siphon.

O. Compressirosta ranges in age from the late Paleocene to the late Pliocene, but is found in Maryland only from 58.7 – 55.8 million years ago. Younger fossils have been found in Virginia, North Carolina, and Florida (I’ve heard a lot of people like to retire there, too).

This guy is also a member of the family Ostreidae, which are known as “true oysters” and the kind that we, as humans, like to dip in cocktail sauce. Unfortunately for us, we will never know what this particular oyster tastes like. Like all oysters, O. Compressirosta is monomyarian, meaning it has only one abductor scar from the muscle used to close the two shells. As you can see from the pictures below it also had irregular shell shape. The shell of this particular fossil would have been composed of calcite that weathered away leaving a sandstone cast like the C. Gigantea found in the same location. Oysters like these reproduce hermaphroditically, possessing both the larva and egg producing abilities sometimes switching between the two based on circumstance.

Try not to think about this post next time you are at Red Lobster. Sorry.

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A few years ago I took a Geology Field Studies class through NVCC. One of the trips we took led us to a creek in Maryland just off the Capital Beltway. I believe it was Henson Creek, but it has been awhile and the memory is a bit foggy. There was no path so we rolled up our pant legs and tromped through the creek, where the glauconitic quartz sandstone, Aquia Greenstone Formation, is exposed nicely. It wasn’t until we had walked in about 150 meters that it was brought to our attention we were walking all over these guys:

That is Cucullaea Gigantea, a now extinct bivalve from the Paleocene ranging in age from 58.7 – 55.8 million years old. These guys were infaunal suspension feeders, meaning they burrowed into the seafloor and strained their food from the water. By now the shell had completely eroded away, but what was left is a perfect mold of the interior composed of the greenstone. When this creature died a small opening was left so that sediments could fill in and later harden into rock. As time passed the more easily weatherable shell went away and all that remained was the more durable greenstone. C. Gigantea is about twice the size of its relatives such as Cucullaea recendens averaging about 8 – 14 cm in length, while C. Recendens averages 5 – 8 cm. Finding marine fossils in a Maryland creek bed also shows that it was once a marine environment. Along with a plethora of C. Gigantea fossils I also found some Ostrea compressirosta fossils, which I will talk about in a future post.

So, if you ever feel like going fossil hunting in the Washington DC area, Henson Creek in MD is bound to get you a handful of these guys

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